Membrane filters made of porous PTFE presently on the market generally have average pore diameters ranging from about 0.1 to 10 .mu.m with the maximum average pore diameter being 10 .mu.m at the most. Recently, however, there is a demand for development of a PTFE membrane filter having a larger average pore diameter. For use as an air filter in clean rooms or the like, membrane filters are required to have low pressure losses. Further, PTFE porous materials are best suited for use as a sheathing material for ultra-high-speed coaxial cables, and the higher the porosity thereof, the higher the performance of the cables.
Among conventional processes for producing PTFE porous materials, a representative method is to heat unsintered formed PTFE containing a liquid lubricant to about 327.degree. C. or more while the PTFE is kept in an at least uniaxially stretched state (as described in JP-B-42-13560). (The term "JP-B" as used herein means an "examined Japanese patent publication".) According to this method, PTFE polymers having average molecular weights (number-average molecular weights) as a high as 3,000,000 or more and showing good stretchability have conventionally been used, and the porosity can be controlled by changing the degree of stretching. This method, however, is defective in that the stretching of unsintered formed PTFE can only be performed at limited degrees of stretching and, hence, the porosity also is heightened in a limited range.
A method has been proposed in which sintered formed PTFE is annealed to heighten crystallinity and then the PTFE is uniaxially stretched, thereby to produce a PTFE porous membrane having micropores (as described in JP-B-53-42794). However, since the stretching cannot be conducted at higher degrees of stretching, membranes having low porosity and poor permeability can only be obtained.
A recently proposed method is to produce a PTFE porous membrane having micropores with a high porosity by use of low molecular weight PTFE that shows poor stretchability (as described in JP-A-64-78823). (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) This method employs a special procedure comprising shaping a PTFE fine powder having a number-average molecular weight of 1,000,000 or less by means of paste extrusion, subjecting the formed PTFE to sintering and then heat treatment to heighten crystallinity, and then stretching the resulting PTFE at least uniaxially. This method, however, is intended to produce porous membranes having small average pore diameters and is limited in heightening porosity.
As described above, there have been no known method which is useful for producing a PTFE porous material having a high porosity and a large average pore diameter.